Yoob is first seen after discovering a group of Yoshis still hiding on Yoshi's Island. He quickly eats all of them, including Yoshi and Kylie Koopa, although Kylie is quickly spat out. Yoob then leaves the area. Later, Yoob is climbing a cliff when two Shroob UFOs appear and enlarge him. Yoob then proceeds to eat Baby Bowser, Mario, Luigi, Baby Mario, and Baby Luigi.

Once inside Yoob's Belly, the group (sans Baby Bowser) helps some of the previously swallowed Yoshis move forward in Yoob's stomach by pushing a Chomp Rock, which clears a path that lets them walk through Yoob's intestines. They then discover that part of Yoob's belly is a factory for trapping Yoshis. The factory, maintained by RC Shroobers, stores Yoshis in Yoob Eggs, then places them on a conveyor belt that takes them to Sunnycide. The group helps the Yoshis push the Chomp Rock away, then find Toadbert, who has forgotten everything that happened after he traveled back in time, but is still taking care of some trapped Yoshis. Toadbert tells them that Sunnycide is guarding the exit, and they go to confront it at the end of Yoob's belly. Sunnycide uses the Yoob Eggs in battle, and likewise, the heroes can free the Yoshis by breaking the eggs. Eventually, they defeat Sunnycide, and everyone, including the Yoshis, Toadbert, and Baby Bowser, escapes Yoob through the digestive tract, while Yoob stops functioning altogether.

If Mario and Luigi return to Yoshi's Island, they can find Yoob's inactive body still on the cliffside. They are able to re-enter Yoob as they wish, though they must exit through the digestive tract again.

The Yoob and his backstory in the Super Mario-Kun volume 36.

In the Super Mario-Kun, Yoob appears in volume 36. Unlike in Mario & Luigi: Partners in Time, Yoob originally was just a giant, carefree, friendly Yoshi who is gluttonous. He is used as a vessel for Sunnycide, as Sunnycide takes control of the giant Yoshi and turns him into a Yoob. After the brothers get eaten, they defeat Sunnycide inside, which reverts the Yoob to his normal state. The giant Yoshi then eats the Cobalt Star Shards, which causes the brothers to panic, and ends the chapter. They manage to get the shards out of him.

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Yoob resembles a pink Yoshi, except it is much larger and fatter (even bigger when the Shroobs zap it), has black spines, a black saddle, blue shoes, green underbelly, black irises, red pupils, and pincers. Yoob bears a loose resemblance to Barney the Dinosaur, the host of Barney & Friends, an educational children's program. As a matter of fact, a Nintendo Power issue predating the release of Mario and Luigi: Partners in Time mentioned the similarities in a pre-release screenshot of the game.[citation needed]

When the babies dig underground or attack with the hammer inside Yoob's Belly, Yoob can be seen suffering in pain on the top screen. He also suffers when the babies open a part in his belly using the buttons and when the Yoshis roll the Chomp Rock down his stomach, creating a path. This stops after Sunnycide is destroyed.

In Mario & Luigi: Bowser's Inside Story, an enemy named Fawful Guy has an attack where it is eaten and turned into an egg by an off-screen creature. The off-screen creature's roar is identical to Yoob's and the egg color is identical to a Yoob Egg.